Variable delay lines are used to delay signals in many different types of circuits, such as delay locked loops (DLLs). Variable delay lines typically include a blender delay and a coarse delay. The coarse delay typically includes a chain of coarse delay elements or units, each coarse delay element delaying an input signal by an equal amount. By selecting the number of coarse delay elements used, a coarse delay value is obtained. The blender delay typically provides a delay between zero and the delay of one coarse delay element. When combined, the blender delay and coarse delay of the variable delay line provide the desired total delay based on a control signal input.
One disadvantage of the blender delay and coarse delay combination is that at the point where the coarse delay is incremented or decremented by one coarse delay element, the blender delay transitions from its minimum delay to its maximum delay or from its maximum delay to its minimum delay. This sudden change in the delay of the blender delay and coarse delay needs to occur during the same phase of the input clock signal to avoid a synchronization problem. The synchronization problem becomes more apparent as the frequency increases. Typically, solutions to the synchronization problem have used multiple delay lines or large multiplexing circuits, both of which increase layout space requirements, current consumption, and costs.